Exposure head for photoelectric colorimeters



April 29, 1958 Original Filed Jan. 18, 1951 Z W i INVENTORL wwww ATTORNEYJ.

A ril 29, 1958 R. s. HUNTER EXPOSURE HEAD FOR PHOTOELECTRIC COLORIMETERS 3 Sheets-Sheet 2 Original Filed Jan. 18, 1951 INVENTOR:

BY W m/am,

ATTORNEYQ April 9 1958 R. s. HUNTER 2,832,257

EXPOSURE HEAD FOR PHOTOELECTRIC COLORIMETERS Original Filed Jan. 18, 1951 3 Sheets-Sheet a INVENTOR! ATTORNEYS.

Unite EXPGSURE HEAD Filih PHUTUELECTRXC IQLGRIMETERSE Richard S, Hunter, Franklin Park, Va., assignor to Henry A. Gardner Laboratory, Inc, Bethesda, Matt, is corporation of the District of Columbia Original application January 18, 1951, Serial No. 2%,6ti5, now Patent No. 2,696,750, dated December 1d, 1954. and this application April 13, 1954, Serial No.

4 Claims. (Cl. Sh -l4) was which constitute coordinates identifying the color by its psychological properties, i. e. by the impression received by an observer who views the color specimen. The prior apparatus included photocell-filter combinations for developing currents which varied in magnitudes with the values of the several tristimulus factors, and manually adjustable potentiometers for measuring the currents by balancing them against the much higher current output of a comparison photocell, the potentiometer dials being graduated in numerical values of the several factors.

Objects of the present invention are to provide cabinets for photoelectric colorimeters which include automatically balancing potentiometers for the measurement of one or more of the factors which identify a color on a tristimulus colorimetry system. Objects are to provide an exposure head for an automatic colorimeter, the exposure head including a light source, supports for a standard specimen and a test specimen, and a plurality of photocells, the positions of certain of the photocells being adjustable for purposes of calibration. Another object is to provide an exposure head including a light source, a transparent support for positioning a test specimen in the path of light rays from said source, a plurality of photocells positioned to receive light rays reflected from a test specimen, and a movable support for a standard specimen, the movable support being adjustable to position the standard specimen in the path of light rays from said source to the transparent support for test specimens.

j which:

Fig. 1 is a schematic view of the optical and electrical elements of an automatic cotton colorimeter embodying the invention;

Fig. 2 is a fragmentary schematic view showing only those elements of the complete colorimeter which are operative during color measurement;

Fig. 3 is a fragmentary schematic view showing only those elements which are operative during intervals between color measurements to standardize or retain the initial calibration of the colorimeter;

Fig. 4 is a circuit diagram of an automatic and self standardizing colorimeter for measurement of the three values which identify a color on a tristimulus basis, the

States atom network being manually adjustable to select the efiective measuring portion of each scale range;

Fig. 5 is a perspective view of a cotton calorimeter which includes apparatus as shown schematically in Fig. 1;

Fig. 6 is a front View of the exposure head of the colorimeter, with some of the parts shown on substantially vertical central section;

Fig. 7 is a transverse vertical section through the same substantially on a line 7--7 of Fig. 6; and

Fig. 8 is an end view of the exposure head.

In Fig. 1 of the drawings, the reference character E identifies a light source for illuminating a test specimen TS from which light is reflected to a current generating photocell PR which is covered by a green filter G, and

' to a photocell combination Pb comprising a pair of photocells P which are covered, respectively, by a blue filter i3 and a green filter G. The photocells P are connected in series opposition, and the relative areas of the photo cells and the transmission characteristics of the filters are such that a net positive current output from the combination is proportional to the yellow color component of the specimen on the same scale that the current output of the photocell PR is a measure of the luminous reflectance R of the specimen. The light source also directly illuminates a comparison photocell CF to develop a con stant current of an order substantially higher than the current outputs of the photocells which are illuminated by reflectance from the test specimen TS. it is to be understood that thespecimen might be such that it transmitted light therethrough to the photocells but, for convenience of description and for the specific case of a cotton colorimeter, the invention will be described on the assumption that light is reflected from the test specimen to the measuring photocells.

Reference is made to the copending application for a detailed explanation of the photoelectric method of measuring the three factors which serve to identify a color on a tristimulus colorimetry system of uniform perciptibility units. in the grading of materials or specimens which have only a limited range of color variation, it is sufiicient to measure only two factors as, for example, luminous reflectance and yellow value in the case of raw cotton. Other materials which vary in color over a limited range may of course be similarly measured in terms of only two of the tristimulus factors which afford a positive identification of any color.

The colorimeter includes two electrical networks for balancing the current outputs of the measuring photocell combinations PR and Pb against fractions of the substantially higher current output of the comparison photocell CP; the networks being energized alternatively by ganged switches for measurement of the color of a specimen TS or for restandardization when a standard specimen 8 of known color is moved into position to intercept the light rays directed towards the test specimen and to reflect them to the measuring photocell units.

Considering first the measuring network which is established when the switches are positioned as shown in Fig. 1, and which is separately shown in Fig. 2, the positive terminal of the comparison photocell CP is connected to a conductor 11 which constitutes the base or voltage reference point of the networks by a jumper 12, and the negative photocell terminal is connected by a lead 12' and a switch S1 to one terminal of the sliclewire R of an automatic potentiometer, the other slidewire terminal being returned to the conductor 11 through the series resistances R1 and R2. The positive terminal of the photocell PR is connected by lead 13 and switch S2 to the tap 14 of the potentiometer slidewire R and the negative terminal is connected by a lead 13' to the junction of the resistances R1 and R2. An electronic ampliassazsr' fier AR is shunted across the photocell PR, and a resistance SR is included in lead 13 between the amplifier connection and the switch S2.

As shown more particularly in Fig. 2, the slidewire b of a second automatic potentiometer is connected in parallel across fixed resistances R1 and R2 and that portion of potentiometer resistance Rd below the motorcontrolled movable tap l i whereby the current established through this slidewire by the comparison photocell CP varies with the luminous reflectance of a specimen. The positive terminal of photocell combination Pb is connected through lead 15, series resistance Sb and switch S3 to the sliding contact 16 of the slidewire b, the negative terminal or the photocell combination is connected to conductor 11 by a lead 35, and an electr amplifier Ab is shunted across the photocell combinatioi in advance of the series resistance Sb.

The measurements of the luminous reflectance R are made in known manner by moving the contact 14 along the slidewire R to divert a small fraction of the cornparison photocell current from its path through slidewirc R and resistors to oppose the current from photocell PR, thereby establishing a voltage balance at the input terminals of the amplifier AR and establishing a comparison current through the slidewire b of a value which is dependent upon the luminous reflectance. At the same time, the Contact 16 of that slidewire is adjusted to establish a potential balance at the input terminals of the amplifier Ab. The amplifiers are of electronic multitube type and energized from the usual light and power lines, not shown, and the amplifiers AR and Ab control the operation of the reversing induction motors MR and Mb respectively which adjust the sliding contacts 14 and 16 respectively. The output terminals of amplifier AR are connected through switch S4 and leads 17 to a field winding F of the motor MR, and the other field winding F is continuously energized from the source AC. Similarly, the amplifier Ab is connected through a switch S and leads 18 to a winding F of motor Mb which has a second winding F.

Pointers l4 and 16 are secured to or otherwise movable with the sliding contacts l4, 16 respectively, and

are displaced at right angles to each other over a color diagram 19 which is preferably translucent and illuminated from the rear by a lamp, not shown. The color diagram may be provided with graduatiens and scales of any desired character and, in the case of a cotton colorimeter, is divided by sets of intersecting lines into zones corresponding to the several classes established by the 1946 international Grade Standards Conference. it is to be noted that, to bring the measuring characteristics of the colorimeter into conformity with the established visual gradations of the surface color of raw cotton or other material, the measuring network must include one or more scale-adjusting elements. For example, the resistors R1 and R2 are connected between the slidewire R of the luminous reflectance measuring circuit and the base conductor 11 to determine the effective measuring range of the luminous reflectance values R The measuring range may be increased by decreasing the in-series resistance below the slidewirc R and, conversely the measuring range may be decreased by increasing the in-series resistance. Uniformity in n'ieasurement oli' chroma b at different reflectance values effected by selecting an appropriate ratio of the total resistance across the comparison photocell (slidewire R plus resistances R1 and R2) to the resistance of the slidewire b. A. decrease in the total resistance across the comparison photocell results in a more rapid variation of the comparison photocell current through the slidewire b for equal adjustments of the contact l4- on the slidewire R and thus results in an increased sensitivity or expanded scale displacement of the chroma I2 pointer 16' at low values of the luminous reflectance of a test specimen. The I; spacing may also be varied by adjustment or selection of "col the value of the series resistance Sb. Closer scale spacings and therefore a longer b-scale range are obtained when the value of resistance Sb is reduced.

Photoelectric measuring circuits in which the current output of a measuring photocell is balanced against a fraction of a constant current of a higher order, for example of an order of times the magnitude of the measuring current, are of high sensitivity and consequently are subject to error from relatively small variations in the operating characteristics of the photocells, the electronic amplifiers and the transmission efficiencies of the several light paths. Continuous calibration or restandardization is therefore essential for accurate measurement of color factors and, in accordance with the invention, the colorimeter includes means continuously operative during intervals between measurements to maintain the standardization or initially established accuracy of the measuring apparatus. The restandardization network includes, in general, manually adjustable potentiometers which are substituted for the automatic potentiometers of the measuring network, and another pair of reversible motors which are switched across the output terminals of the respective amplifiers and which adjust the positions of certain of the photocells with respect to the light source in such manner as to establish voltage balances at the input terminals of the amplifiers when the potentiometer contacts are set at the known reflectance and chroma values of a standard specimen S.

The change-over between the measuring and restandardizing condition of the colorimeter is efiected by shifting the series of switches S1 to S5 from the end positions as shown in Figs. 1 and 2 to the alternative positions as shown in Fig. 3. The several switches are ganged for simultaneous operation, as indicated schematically by the broken line 21 connecting the blades of switches S1 and S2, and by the legend of Fig. l, and are preferably elements of a telephone type relay including a solenoid 22. As shown schematically in Fig. 1, the armature 23 of the relay is biased towards dropout by a spring 24, and the several switches are therefore normally in the Fig. 3 positions to condition the colorimeter for restandardization. The solenoid 22 is energized through a lead 25 and a normally open switch 26 from a current source 27 which, for simplicity of the Fig. 1 diagram, is shown schematically as a battery 27 having a grounded terminal. In practice, this current source 27 is preferably the alternating current source which energizes the amplifiers, the motors and the light sources of the colorimeter. The closing of the switch 26 to condition the colorimeter for a measuring operation also completes the energizing circuit of a solenoid 28 to withdraw the standard specimen S from its normal position between the light source E and a test specimen TS; the specimen being returned to normal position by a spring 29 when the switch is opened.

As best seen in Fig. 3, when the switch 26 is open and the solenoids 22 and 28 are deenergized, the comparison photocell CP is connected by switch S1 across the resistance CR of a manually adjustable potentiometer having a tap 31 which is connected by lead 32 to the standardizing contact of the switch S2 and, through the resistance Cb of a second manually adjustable potentiometer, to the conductor 11. The blade of switch S2 is' connected to the positive lead 13 from the reflectance-measuring photocell PR and a fraction of the comparison photocell current is thus diverted from the lower section of the potentiometer resistance CR to oppose the current output of the reflectance-measuring photocell PR. The tap 33 of the potentiometer resistance Cb is connected through switch S3 to the lead 15 from the photocell combination Pb, and the switches S4 and S5 of the amplifiers AR and Ab complete circuits through leads 34, 35 respectively to windings F of reversible motors M1 and M2 which each have continuously energized windings F. The com parison photocell CP is mounted on a hinged or flexible supporting plate 36 which is urged in one direction by a spring 37 and is moved in the opposite direction by motor M1 to adjust the comparison photocell CP into such position, with respect to the light source E, that a voltage balance is obtained at the input terminals of the amplifier AR when the tap 31 of the calibrating potentiometer CR is set at the known luminous reflectance value of the standard specimen S. Similarly, the green photocell P of the photocell combination Pb is mounted upon a hinged or flexible plate 38 which is urged in one direction by a spring 39 and is positively moved in the opposite direction by the motor M2 to establish a voltage balance at the input terminals of the amplifier Ab when the tap 33 is set on the chroma calibrating potentiometer Cb at the known chroma of the standard specimen S.

The power switch for energizing the colorimeter should preferably be closed for a sufficient time to establish stable temperature conditions at the photocells and stable operating characteristics for the amplifiers, for example a quarter-hour or more, before measurements are made on test specimens. Closely accurate measurements of color values may of course be had without this delay since the colorimeter is continuously in standardizing condition until the switch 26 is closed for a color measurement, and the automatic potentiometers operate quickly to complete a measurement. It is preferable, however, to delay all measurements until at least relatively stable operating conditions are established.

The cotton colorimeter is of high sensitivity as the range of cotton color is limited and the full measuring ranges of the automatic potentiometers are available to cover only portions of the 0-100 ranges of R and b values. The same high measuring sensitivity over the entire ranges of reflectance and chroma variation may be had with a colorimeter, as shown in Fig. 4, in which the measuring ranges of the otentiometers are limited but may be manually selected .to cover any desired portion of their individual scale ranges. The Fig. 4 colorimeter difiers from that shown in Fig. 1 by the inclusion of a photocell combination Pa and polarity-reversing switch for measurement of red-green values, a polarity-reversing switch associated with the photocell combination Pb for measurement of blue values, and the circuit elements for shifting the measuring ranges of the otentiometers. Except for the addition of the photocell combination Pa, the Fig. 4 network is basically the same as that of Fig. 1 and such elements as are or may be identical with those of Fig. l are identified by corresponding reference numerals but will not be described in detail.

The slidewire resistance R of the automatic potentiometer for measurement of the luminous reflectance of a test specimen is tapped across and manually slidable along a pair of identical resistances R, R" whose outer terminals are connected respectively to the measuring contact of switch S1 and to the conductor 11, the arrangement being such that the total series resistance is constant and independent of the adjustment of the slidewire along the resistances. The effective measuring range of the potentiometer may therefore be shifted over the full range of R values by an appropriate adjustment of the slidewire. Similarly, the slidewire b of the automatic potentiometer for measurement of yellow-blue values is tapped across and slidable along a pair of similar re sistances b, b whose outer terminals are connected respectively to the positive lead 13 from the photocell PR and the conductor 11. A polarity reversing switch 41 is provided between the photocell combination Pb and its leads 15, as the polarity of the current output reverses over the yellow-blue range, and the current introduced into the slidewire b from the photocell combination Pb must be of opposite polarity to the comparison photocell current through the slidewi re.

The apparatus and network for measurement of the tristimulus factor ia=red-green is, in general, similar to the apparatus and network for measurement of :b, and the :a network is effectively connected in parallel with the 1th network. The photocell combination Pa includes a photocell P covered by a filter comprising a blue section B and a smaller amber section A, and a second photocell P covered by a green filter G and connected in whose outer terminals are connected to lead 13 and the conductor 11 respectively. The sliding contact 44 of the potentiometer slidewire a is connected to the measuring contact switch S6, and is adjustable by a motor Ma having a field winding F connected across the output terminals of the amplifier Aa by a switch S7 which is ganged to switches S1 to S6. The resistance Ca of a calibrating potentiometer is connected in parallel with the calibrating potentiometer resistance Cb by a lead db, and the manually adjustable contact d6 of resistance Ca is connected by lead 47 to the standardizing contact of the switch se. When the ganged switches S1 to S? are adjusted for standardization, the output terminals of amplifier Au. are connected through switch S7 to a reversing motor M3 which adjusts the hinged support 48 of the green photocell P of the combination Pa, in opposition to a spring 49, to effect a voltage balance at the input terminals of the amplifier Aer.

The effective of each automatic potentiometer depends upon the relative magnitudes of the potentiometer resistance and of the resistances across which it is tapped. The effective measuring range will be about or of the full scale range when the resistance of the potentiometer slidewire (R at or b) is or ohms, respectively, and the associated series resistances (R, R" b a) each has avalue of 200 ohms.

A cotton colorimeter as shown schematically in Fig l is preferably self-contained in a movable cabinet 51 with a removable top 53 at about table or desk height, the top having an opening adjacent the left side through which the cover glass 55 of the exposure head extends to lie parallel to and slightly above the top surface, see Fig. 5. The top 53 is provided with a further opening across which a transparent or translucent plate 56 is secured flush with the cabinet top surface. The color diagram 19 is appropriately positioned on the plate 56 and retained thereon by a relatively heavy and transparent plastic sheet 5'7 which, in turn, is retained on thetop 53 by a frame 58. The pointers 1d and 16' of the R and b potentiometers move in horizontal planes closely adjacent the lower face of the plate 56, and the shadows 14" and lb" of the pointers are thrown upon the plate 56 by a lamp, not shown, within the cabinet 51. The exposure head of the colorimeter is located at the left side of the cabinet 51, and a door 59 is hinged to the left side wall of the cabinet for access to the exposure head. The colorimeter is intended for use in cotton classing rooms where lint is always present, and gaskets are provided in conventional manner to seal the cabinet interior against the entrance of lint and dust. The switch 26 for energizing solenoid 22 for actuating the changeover switches and the solenoid 28 for withdrawing the standard specimen S from beneath the exposure area glass 55 is preferably a foot-pedal switch.

The exposure head which houses the optical and photoelectiica'l elements of the colorimeter is a sheet metal box of rectangular form with an open front facing the cabinet door 59. Light sources E, E are located at opposite ends of the exposure head for directing light beams upon the exposure area of cover glass 55 at angles of and the several measuring photocells are positioned to receive light reflected from the specimens in a near-perpendicular direction. i i

The main housing member 61 is open at its top central section, and a partition wall is secured within this housing and comprises a lower U-shaped section 62, intermediate portions 63, 63 flaring outwardly therefrom at and terminal sections 64, 64 which extend somewhat above the top wall of the main housing member 61. The open top of the housing 61 is closed by a flanged cover plate 65 which fits over and is secured to the projecting ends 64, 64' of the partition wall and to the housing 61. The cover glass is secured over an opening in the plate 65 by a frame, and the standard specimen S is supported upon a shaft 67 journalled on the plate 65 and carrying a gear 63 in mesh with a toothed rack on a rod 69 secured to the core 28 of the solenoid 28. The toothed rod 69 is yieldingly urged away from the solenoid by spring 29 to hold the standard specimen S in position im mediately beneath the glass cover plate 55. The solenoid 28 is supported by and housed within a box 71 secured to the rear face of the cover plate 65.

The light sources E, E are sealed beam automobile spot lights mounted in plates '72, 72 secured by flexible extensions 73, 73 thereof to light shields 74, 74' at the opposite ends of the main housing member 61. The upper ends 75, '75 of the mounting plates are parallel to the end walls of the housing 61 and screws 76, 76" extend through the housing wall and the ends 75, to adjust the light source positions in opposition to springs 77, 77" which surround the screws 76, 7 6' respectively. The axes of the light sources E, E are at 45 to the cover glass 55 of the exposure head, and the central portions 0 of the beams from the sources E, E are directed upwardly through openings in the partition wall portions 63, 63' to the lower surface of the standard specimen S or, when it is withdrawn, to the glass 55 of the exposure head. Edge portions d and e of the light beams are intercepted and reflected to a pair of parallel connected comparison photocells CP, CP' by mirrors 78, '79 and 78', '79, respectively. Filters 84? may be provided over the comparison photocells for protection against overheating by the incident light energy and/or to determine the effective output current of the comparison photocell combination. The mirrors 78, 78 are supported on brackets 31, 81' secured to the housing 61 and having extensions 82, 82 respectively, which extend along the light beams c, c and constitute light shields. The mirrors 79, 79 are supported on light shield members 33, .83 which are secured to the partition wall 62 and extend downwardly to overlap the light shields 74, 74 respectively.

The comparison photocells Cl, CP are supported upon a bent plate or bracket 36 which is secured to the back wall of the housing by a hinge 84. The plate 36' is urged downwardly by the spring 37, and its position is controlled by the reversible motor M1 which has a shaft with axially elongated gear teeth 85 in mesh with a pinion 86 on a shaft 87 threaded through a stationary nut or block fi The motor and nut 83 are supported upon a bracket 89 which is secured to the rear face of the back wall of the housing 61, and the end of the shaft 87 projects through the back wall to bear against the hinged plate 36'.

An apertured wall 91 extends horizontally across the U-shaped section 62 of the partition wall to support the several measuring photocells. The Pb combination differs from that shown schematically in Figs. 1 to 3 in that there are two photocells P with blue filters B at the upper face of the wall 91, and a third photocell P with a green filter G on a plate 38 hinged to the lower face of wall 91 and urged downwardly by the spring 39. The motor M2 controls the position of the green photocell of the Pb combination through mechanism as above described with respect to the motor M1 and identified by corresponding but primed reference numerals. The photocell PR which is covered by a green filter G and develops a current proportional to the luminous reflectance of a specimen is also mounted on a plate 92 which is hinged to and cells.

belowthe wall 91, the plate 'being urged downwardly by a spring 93 and adjustable, as a factory adjustment, upwardly by a screw 94 which is threaded through anut 95 welded orotherwise fixed to the Wall of the housing 61.

Each end of the housing 61 is provided with ventilating openings 96 which are so positioned with respect to the light shields 74, 81- 83 and 74', 81'-83 that stray light from the amplifier tubes and from the lamp which i1luminates the color diagram '19 can not reach any of the photo- The circuit connections of the light sources E, E and the several photocells are not shown in Figs. 6 and 7 as they would tend to confuse the illustration of the physical structure of the exposure head and could add nothing to the clarity of the circuit diagrams of Figs. 1 to 3. Conrentional cable connections, not shown, are preferably'employcd to connect the exposure head unit to other units of the complete colorimeter.

it is to be noted that while the connection of the slidewire I; of the chroma potentiometer between the sliding contact 14 of slidewire R Figs. 1 and 2, provides uniform measurements of 11 values, on a uniform scale of graduations, from high to low values of reflectance R the scale graduations of R values are not uniform but are expanded towards its upper end. A uniform scale of R values may be had by gauging a second potentiometer (corresponding to potentiometer R Fig. 2 of Patent No. 2,574,264) in parallel with the potentiometer R to supply the slidewire b with a current derived from the comparison photocell CP and varying in magnitude as a function of the reflectance of the specimen.

It is therefore to be understood that the invention is not limited to theparticular photoelectric circuits and apparatus herein illustrated and described as various modifications which may occur to those familiar with the'art fall within the spirit and scope of the invention asset forth in the following claims.

This application is a division of my application Serial No. 206,685, filed January 18, 1951, now Patent No. 2,696,750 granted December 14, 1954.

I claim:

1. An' exposure head for a photoelectric calorimeter, said exposure head comprising a rectangular housing, light sources at opposite ends of said housing for directing collimated light beams towards the upper central portion of said housing at angles of substantially 45 a cover plate over the upper central portion of said housing and carrying a cover glass on which a test specimen may be positioned, a partition wall within said housing intermediate said cover plate and said light sources including a central Well-shaped section and apertured sections adjacent thereto to permit the passage of axial portions of said light beams upwardly therethrough to be directed upon said cover glass, an apertured partition member extending across said partition wall well-shaped section, measuring means including a plurality of photocells mounted on said partition member to receive light reflected from the specimen, means supporting one of said photocells for angular movement with respect to said partition member, spring means urging said supporting means in one direction, a screw for adjusting said supporting means, comparison photocell means associated with each of said light sources and positioned within said housing beneath said well-shaped section, and mirrors for reflecting to each of said comparison photocell means the edge portions of the light beams from the associated light sources.

2. An exposure head as defined in claim 1 wherein said screw is threaded through said housing and is manually adjustable.

3. An exposure head as defined in claim 1, in combination with means supporting a second of the photocells of the measuring means for angular movement with respect to the partition member, a spring biasing said second photocell for movement in one direction, a screw extending through said housing and constituting a stop for limiting movement of said second photocell by said spring, and a reversible motor for adjusting said screw.

4. An exposure head as defined in claim 1 and further including a hinged support on which said comparison photocell means are mounted, and means including a reversible motor supported on said housing for auto matically adjusting said hinged support of said comparison photocell means with respect to said light sources in accordance with the voltages produced by said measuring photocell means.

References Cited in the file of this patent UNITED STATES PATENTS Rose Mar. 9, Holven et a1 Feb. 17, Benning May 15, Scott Aug. 20, Pierson Sept. 27, Hunter Sept. 27, Schnelle Aug. 18, Hunter Mar. 20,

FOREIGN PATENTS Germany June 30, 

